FeLV
FeLVs are a group of contagious oncogenic RNA viruses that cause both neoplastic and non-neoplastic diseases in cats. FeLV infections are the main cause of disease-related deaths in cats. During FeLV replication, a DNA copy of the viral RNA genome is made and inserted into the DNA of infected cells. The integrated FeLV DNA codes for the replication of more virus which is shed from infected cells. By recombining with the host's genes to generate feline sarcoma virus (FeSV), the virus can cause cell transformation.
The FeLV genome is a 60-70S single stranded RNA consisting of a gag gene that encodes the internal viral proteins, a pol gene that codes for the viral RNA dependent DNA polymerase (reverse transcriptase), and an env gene that encodes the viral envelope proteins gp70 and p15E. Viral interference and neutralization tests have shown that there are three subgroups of envelope antigens (designated A, B and C) that are similar but distinct from each other and give rise to the three recognized FeLV subgroups (also designated A, B and C).
The fate of a cat that is exposed to FeLV will depend upon its immune response to the FeLV--particularly to the FeLV envelope antigens. Studies have shown that about 40% of the exposed population produce high titers of anti-FeLV envelope antibodies and become immune, about 30% do not respond adequately and become persistently infected, and about 30% are neither infected nor immunized but remain susceptible. The fact that a significant portion of the exposed population acquires natural immunity has led investigators to try a variety of materials as vaccines against FeLV. Inactivated virus was found to be ineffective except at high doses. Purified gp70 was also reported to be generally ineffective as a vaccine. Killed tumor cells have been reported to be successful in preventing leukemia but not viremia. A soluble tumor cell antigen obtained from the tissue culture medium of an FeLV transformed cell line (FL-74) was also tried and found effective in preventing the induction of FeLV virus infection.
Microbially Produced Vaccines
Science (1981) 214:1125-1128 and British patent application No. GB 2103622 A describe microbially-produced polypeptides that are useful as foot-and-mouth disease (FMDV) vaccines. Complementary DNA (cDNA) fragments were prepared from the portion of the FMDV genome that codes for the capsid protein VP.sub.3. These cDNA inserts were incorporated into a bacterial plasmid containing a tryptophan (trp) promoter and the recombinant plasmid was used to transform competent E. coli. The recombinant bacteria expressed a fusion protein composed of a polypeptide encoded by the trp leader and a portion of the trp E gene and a polypeptide encoded by the VP.sub.3 insert. The fusion protein was purified and tested in vitro and in vivo for its ability to bind anti-VP.sub.3 antibodies and its ability to prevent FMDV infection in livestock. Science (1981) 219:614-620 describes the synthesis of a surface protein of rabies virus via expression in recombinant E. coli of a gene derived from the rabies virus genome.
Vaccination Procedure
Emini, et al, Nature (1981) 304:699-703 report that a low subinfectious and subprotective dose of live poliovirus can be used to potentiate the immune response to synthetic peptides derived from poliovirus. It appears as if the initial synthetic vaccine "primes" the immune system and that the subsequent viral boost allows the initially-primed immune response to manifest itself in a potent humoral response. Other investigators have reported similar findings using microbially produced (recombinant) vaccines. These include poliovirus and hepatitis B (prime with recombinant/boost with live virus) and foot-and-mouth disease virus (prime with recombinant/boost with killed virus).